Method of liquid developing a photoconductive plate

ABSTRACT

AN IMPROVED ELECTROPHOTOGRAPHIC IMAGE PRODUCING PROCESS CAPABLE OF REPRODUCING A HIGHLY RESOLVED CONTINUOUS-TONE IMAGE. THE PROCESS UTILIZES AN ELECTROPHOTOGRAPHIC PLATE HAVING AN INSULATING BACK LAYER. THE PROCESS IS CHARACTERIZED IN THAT A UNIFROM CHARGE OF THE SAME POLARITY IS GIVEN TO BOTH THE TOP SURFACE AND THE BACK SURFACE OF THE ELECTROPHOTOGRAPHIC PLATE.

United States Patent US. Cl. 961 4 Claims ABSTRACT OF THE DISCLOSURE An improved electrophotographic image producing process capable of reproducing a highly resolved continuous-tone image. The process utilizes an electrophotographic plate having an insulating back layer. The process is characterized in that a uniform charge of the same polarity is given to both the top surface and the back surface of the electrophotographic plate.

This invention relates to a process for producing a visible image on a surface of an electrophotographic plate.

PRIOR ART The electrostatic image in electrophotography is usually formed on the surface of a photosensitive member consisting of a photoconductive insulating layer bonded to a conductive layer. This is achieved by first producing a uniform distribution of electrical charge on the surface of the photoconductive insulating layer and then exposing the photoconductive insulating layer to an optical image.

The photoconductive insulating layer has electrical conductivity at the areas which are subjected to light illumination. These areas having electrical conductivity dissipate the surface charges to the conductive layer through the photoconductive insulating layer and form an electrostatic latent image.

Such an electrostatic latent image is usually developed by the electrical attraction of fine toner particles to the latent image areas on the photoconductive insulating layer. Uncharged areas of the surface of the layer do not attract developer particles, and the amount of particles attracted to charged areas is proportional to the intensity of electrical fields associated with the image. The contour of the electrical field in the region of the electrostatic image is an important factor in electrophotographic developing techniques. At the boundary of a line image, there exists a sharp voltage contrast or greater field intensity which is called fringing field. If this image-field configuration is developed by applying charged particles to the layer surface by using the normal liquid developing technique, the small area would be developed out as a solid black, but the larger area would be solid black only at the edges. In the central portion of a large solid image area, the voltage contrasts between adjacent points are small and particle deposition is proportionally reduced. For the reason mentioned above, it has been difficult to obtain an exact reproduction of solid blacks and continuous tone graduation.

To overcome this ditficulty, some developing techniques have been proposed. One of them is the halftone screening method such as those used in the photomechanical operation of lithography. Such a half-toning method is based on the tone graduation due to a variation in the dot size. With this method suificient range of tones is developed for good halftone pictures, but it is impossible to produce completely black areas. Another is the development electrode method.

The edge effects can be eliminated with a development ice electrode. In its simplest form this can be a metallic sheet which is held close to and parallel to the surface of the electrophotographic plate and electrically shorted to the conductive back layer of the plate. The effect of this sheet electrode is to change the field configuration of the electrostatic image and to increase the field in the space above large solid areas of charge. Under these conditions, the various parts of an electrostatic image are developed out nearly in proportion to the electrical charge density on the plate surface because the field strength at any point between the plate surface and the development electrode is proportional to the plate potential. Thus, this development electrode method makes it possible to obtain a high quality of continuous tone and the reproduction of solid blacks.

The field strength between the plate surface and the development electrode is inversely proportional to the distance between the plate and the electrode. Thus it is necessary for effective achievement that the development electrode be placed as close to the plate as possible. The shorter the distance between the photographic plate and the development electrode, however, results in more difficulty in supplying the developer to the surface of the electrophotographic plate.

An object of this invention is to provide an improved electrophotographic image producing process for reproducing continuous-tone images.

Another object of this invention is to provide an improved electrophotographic image producing process capable of reproducing continous-tone images by 'a single charging device.

A further object is to provide an improved electrophotographic process capable of reproducing highly resolved continous-tone images.

A further object is to provide an improved process capable of reproducing highly resolved images, and which is especially suitable for microfilm reproduction.

A further object of this invention is to provide an improved process to control the quality of visible images.

These and other objects of this invention will be apparent upon consideration of the following detailed description taken together with accompanying drawings wherein;

FIG. 1 is a cross sectional view of an electrophotographic plate.

FIG. 2 is a cross sectional view of another electrophotographic plate.

FIGS. 3A-3D are partially schematic representations of the steps in carrying out one embodiment of this invention.

FIGS. 4A-4C are partially schematic representations of the steps in carrying out another embodiment of this invention.

These objects and other advantages are accomplished in accordance with this invention which provides an improved electrophotographic process comprising the step of giving both the top surface and back surface of an electrophotographic plate a uniform charge. The electrophotographic plate. used in the process mentioned above comprises a photoconductive insulating top layer, a conductive layer and an insulating back layer.

It is necessary that the insulating back layer herein described hold an electrostatic charge in the dark or under light conditions for a period at least longer than that necessary for completion of developing process.

The novel process according to the present invention comprises the steps of (1) giving both the top surface of the photoconductive insulating layer and the back surface of the insulating layer a uniform charge of similar polarity with tWo charging devices at the same time; (2) exposing said top surface to a light image; and (3) developing the electrostatic latent image on said top surface by 3 immersing the electrophotographic plate in the liquid developer.

In the process mentioned above, the quality of the developed image may be controlled by a modification of the structure of the electrophotographic plate.

Wtih a slight modification of the foregoing process the quality of the visible image was controlled and a desirable image was obtained on the surface of the electrophotographic plate.

FIG. 1 shows the structure of electrophotographic plate consisting of insulating back layer 11, a conductive layer 12 and a photoconductive insulating top layer 13.

The insulating back layer 11 comprises a layer of material which is electrically insulating both in the dark and under light conditions. It may be made of any suitable and available insulating material such as polyethylene terephthalate, cellulose diacetate, cellulose triacetate, or other film-forming insulating materials.

It is important for achievement of continuous tone image reproduction that the insulating back layer 11 be thicker than other layers 12 and 13. Layer 11 may be opaque when the resultant image is viewed by reflection, otherwise it is transparent.

For illustrative purposes only, layer 11 is preferably a polyethylene terephthalate sheet film of 75 microns in thickness.

The conductive layer 12 comprises, in accordance with conventional electrophotographic usage, a thin film of, for example, aluminum, brass, titanium, copper, copper iodide, other metals, metal salt, glass with a transparent or other conductive coating, or like known layer.

Layer 13 may comprise the various photoconductive insulating materials known to be useful in the art of electrophotography. For example, selenium, sulfur or anthracene and other organic photoconductors as well as dispersions of photoconductive pigments such as zinc oxide in various resins or other electrically insulating binder materials may be used. Layer 13 is generally a good electric insulator capable of maintaining a surface charge in the dark, but becomes substantially more conductive when illuminated by visible light, X-rays or other forms of activatign radiation. For illustrative purpose only, layer 13 is preferably a layer of brominated poly-n-vinylcarbazole of 10 microns in thickness.

FIG. 2 shows the structure of another electrophotographic plate consisting of insulating back layer 21, conductive layer 22, insulating intermediate layer 23 and a photoconductive insulating top layer 24.

The insulating back layer 21 comprises a layer of the same material as the insulating back layer 11 in FIG. 1. The layer 22 comprises a layer of the same material as the conductive layer 12 in FIG. 1.

The layer 23 may comprise the various insulating mate rials such as polyvinyl acetate, vinylidene chloride/acrylonitrile acetate maleic anhydride terpolymer, or other electrically insulating materials.

An insulating material having adhesive properties is especially advantageous for use in the insulating intermediate layer 23.

For illustrative purpose only, the layer 23 is preferably a layer of polyvinyl acetate 5 microns in thickness. The layer 24 comprises a layer of the same material as the photoconductive insulating top layer 13 in FIG. 1.

When the photographic process according to the present invention is employed, the resolution power of the developed image on the electrophotographic plate 20 having the intermediate insulating layer 23 between the photoconductive top layer 24 and the conductive layer 22 is higher than that of developed image on the three layered electrophotographic plate 10.

The resolution power depends on the thickness of the intermediate insulating layer 23.

It is preferable for reproduction of a highly resolved and continuous-tone image on the electrophotographic plate 20 having intermediate insulating layer 23 that the thickness of the intermediate insulating layer 23 be less than the thickness of photoconductive top layer 24 and that the thickness of the insulating back layer 21 be more than 10 times that of the intermediate insulating layer.

FIGS. 3A-3D illustrate schematically the process steps for carrying out one embodiment of the invention. As shown in FIG. 3A plate 10 is electrostatically charged by a corona charging device 31 which is connected to a high voltage power supply 32.

The conductive layer 12 is connected to the grounded terminal of the power supply. Corona charging devices are well known in the electrophotography art. Other available methods of applying a uniform potential onto a surface of photoconductive insulating layer 13 or insulating back layer 11 may be employed.

The next step is exposure of plate 10 to a light image as illustrated in FIG. 3B. If the insulating back layer 11 and the conductive layer 12 are transparent, exposure may be made through the insulating back layer 11. Exposure may be made by means of a photographic enlarger 35, as illustrated, or in a camera, or by contact exposure or other means. When illuminated by light, photoconductive insulating layer 13 becomes electrically conductive and permits the charges of the top surface 14 of the photoconductive insulating layer 13 to dissipate through the photoconductive insulating layer 13 to the conductive layer 12. Thus the electrostatic latent image is formed.

However, the quality of visible image produced without the following process is poor because the field strength is too dominant at the boundary of a solid black image. The next step, as shown in FIG. 3C, is to give the back surface 15 of insulating back layer 11 a uniform charge the sign of which is similar to that given to the top surface 14 of the photoconductive insulating top layer 13. It is preferable for reproduction of continuous-tone image that the surface potential of the back surface 15 of the electrophotographic plate 10 be higher than two times the mean value of the surface potential on the top surface 14 after exposure.

The next step, as shown in FIG. 3D, is to develop the electrostatic image in order to produce a visible image by immersing the plate in the liquid developer. Then a continuous-tone image is easily obtained.

It is difficult to reproduce the highly resolved and continuous-tone image when a uniform charge is not given to the insulating back layer 11.

It is, however, easy to reproduce a highly resolved and continuous-tone image when a uniform charge is given to the insulating back layer 11. The resolution power of the developed image depends on the concentration of the toner particles in the carrier liquid. It is preferable for reproduction of the highly resolved and continuous-tone image that the concentration of the toner particles in the carrier liquid be between 0.5 mg./cc. land 5 mg./cc.

FIGS. 4A-4C illustrate schematically the process steps for another embodiment of the invention. When the resistivity of insulating back layer 11 is sufliciently high, a uniform electrostatic charge may be given to both top surface 14 and back surface 15 of the electrophotographic plate 10 at the same time and may also be maintained for a time period necessary for completion of the developing process.

As shown in FIG. 4A, the top surface 14 and back surface 15 of the electrophotographic plate 10 are electrostatically charged by corona charging devices 41 and 42 which are connected to a high voltage power supply 32. Corona charging devices are well known in the electrophotography art.

The next step is exposure of plate 10 to a light image as illustrated in FIG. 4B.

The next step as shown in FIG. 4C is to develop the electrostatic image in order to produce a visible image by immersing the plate 10 in the liquid developer. Then continuous-tone images are easily obtained;

While the mechanisms of producing the continuoustone image are not completely understood, a theory has been proposed which appears to account for the observed phenomena.

A positive uniform charge is given to the top surface 14 of the photoconductive insulating layer 13 in the first step, and then a positive uniform charge is given to the back surface 15 of the insulating back layer 11 in the second charging step. A negative charge is induced on the conductive surface 12 of the electrophotographic plate and then the insulating back layer 11 may be regarded as an electric double layer.

Near the top surface 14 of the electrophotographic plate 10, there exist two electric fields e.g. the electric field produced by the positive charge on the top surface 14 of photoconductive insulating top layer 12 and the electric field produced by the electric double layer mentioned above. According to electrostatics theory, the field strength near an electric double layer depends on the thickness of the layer thereof and it increases as the thickness increases. The charged photoconductive insulating layer may be regarded as an electric double layer but the field strength produced by such a double layer is small at the outside of electrophotographic plate 10.

However, the field strength produced by the electric double layer formed on the insulating back layer 11 is strong because the insulating back layer 11 is thicker than the photoconductive insulating layer 13.

The positive charge on the surface of insulating back layer 11 and the induced negative charge on the conductive layer 12 are only slightly reduced in dark and under light conditions because the insulating back layer 11 is not photoconductive and has high resistivity. Therefore, the field produced by such an electric double layer is only slightly changed during the image producing process. In the light exposed area the positive charge given to the photoconductive top layer 13 is dissipated or reduced in proportion to the exposure. Consequently, the field produced by the charge on the top surface 14 of the photoconductive top layer 13 is reduced in proportion to the exposure. In the non-exposed area the positive charges on the photoconductive top layer remains unchanged, and the field produced by these charges is unchanged.

So the residual field strength near the top surface 14 is very strong. If the electrophotographic plate is immersed in the liquid developer including positive charged toner particles and carrier liquid, the field produced by the electric double layer causes the toner particles to move toward the top surface 14 of the electrophotographic plate 10 and the field produced by the positive surface charge on the top surface 14 causes the toner particles to move away from the image surface. The toner particles are attracted to the top surface 14 when the former field strength is dominant; the toner particles are repulsed from the top surface 14 when the latter field is dominant.

Thus, the toner particles are moved to the top surface 14 and deposited on the light exposed area of the said top surface 14. This area becomes black and corresponds to the white area of the original picture. Consequently, reverse development is carried out.

Since such a situation is similar to that of the case where a development electrode is used, a continuous-tone image is available with this process. In addition, the toner particles which have a positive charge do not deposit on the back surface 15 of the electrophotographic plate 10 because the back surface 15 of the insulating back layer 11 is charged positively. It means that a fog-free image is available on the transparent electrophotographic plate.

When a negative charge is given to the top surface 14 of electrophotographic plate 10 and negative charged toner particles are used, the same effect as when a positive charge is given to the top surface 14 is observed and continuous-tone reversal images may also be obtained.

When a negative charge is given to the top surface 14 and the back surface 15 of electrophotographic plate 10 and positive charged toner particles are used. the field produced by the electric double layer causes the toner particles to move away from the top surface 14 in the exposed area and a fog-free image is available. In such a case, however, it is required that toner particles be supplied to only the top surface 14 of the electrophotographic plate 10 in the developing process.

The invention is illustrated by the following examples. It is to be understood, however, that no limitations are placed on the invention by such examples.

EXAMPLE I Referring again to FIG. 1, the insulating back layer 11 was made of polyethylene terephthalate sheet film of microns in thickness.

A thin layer of copper iodide, as the conductive layer 12, was deposited to the said insulating back layer 11 by using a usual conventional evaporating method.

The layer of copper iodide was thiner than one micron.

1 gram of brominated poly-N-vinyl carbazole, 0.1 gram of chlorinated aliphatic acid ester (Commercially available as Adecasizer 8-3), 0.2 gram of epoxyresin (Commercially available as Epon 828), 0.5 gram of poly carbonate resin (Commercially available as Panlite-C) and 0.003 gram of 2.-(isopropyl-phenylbutadienyl) benzopyrylium perchlorate were dissolved in a mixedsolvent of 8 milliliters of chlorobenzene and 2 milliliters of dichloroethane.

This solution was applied to the said conductive layer 12 by means of a blade coating method and dried to form a photoconductive insulating top layer 13 of about 8 microns in thickness. A three layered electrophotographic plate 10 was thus formed.

The top surface 14 of the plate 10 was positively charged to a uniform potential of +900 volts. The back surface 15 of the plate 10 was also positively chargred and the surface potential became +1500 volts. Then the top surface 14 of the plate 10 was exposed to the light image.

After exposure, the mean surface potential of the top surface 14 of the plate 10 was about +600 volts.

The electrophotographic plate 10 was developed with a liquid developer which included positive charged toner particles and a carrier liquid.

The concentration of the toner particles in the liquid was 1.5 mg./cc. and developing time was 1 second.

A resolution power higher than lines/mm. was available and a good continuous-tone image suitable for micro film reproduction was easily produced.

EXAMPLE II As shown in FIG. 2, polyethylene terephthalate sheet film of 75 microns in thickness was used for the insulating back layer 21. A thin layer of copper iodide for use as the conductive layer 22 of FIG. 2 was deposited on the said insulating back layer 21 as described in Example I. The layer of copper iodide was thinner than one micron. The said conductive layer 22 was coated with polyvinyl acetate by means of a blade coating method.

The layer of polyvinyl acetate formed the intermediate insulating layer 23.

A solution of the photoconductive material described in the Example I was applied to said intermediate insulating layer 23 by means of a blade coating method and dried to form a photoconductive insulating top layer 24 of about 8 microns in thickness. Thus a four layered electrophotographic plate 20 of FIG. 2 was formed.

A series of electrophotographic plates with layers 23 of different thicknesses were prepared. The different layers 23 had thicknesses between 1.5 microns and 14 microns.

The top surface 25 and the back surface 26 of the plate 20 were charged and the plate 20 was developed as described in Example I.

The resolution power of the developed image depended on the thickness of the intermediate insulating layer 23 and the highest resolution power was obtained with an electrophotographic plate 20 having the intermediate insulating layer 23 between 2 and 3 microns in thickness.

EXAMPLE III An electrophotographic plate 20 having the intermediate insulating layer 23 of microns in thickness as described in Example II was used.

The said plate was charged, exposed and developed as described in Example II.

The highest resolution power was obtained when the plate 20 was developed for 1 second with toner particle concentration of 1.5 mg./cc. in the carrier liquid.

EXAMPLE IV The same electrophotographic plate 20 as described in Example III was used.

The same image producing process as described in Example III was carried out.

The concentration of toner particle in the liquid carrier was 5 mg./cc. The y-value of the continuous-tone image was 0.35 with a developing time of 1 second. It increased as the developing time increased and became 1.35 with a developing time of 10 seconds.

What is claimed is:

1. A method of liquid developing a photoconductive plate for producing a visible image on the top surface of said photoconductive plate which includes a photoconductive insulating top layer, a ground conductive layer, and an insulating back layer which is thicker than said other layers, comprising the steps of: giving both the top surface of said photoconductive insulating top layer and the back surface of said insulating back layer a uniform electrostatic charge of the same polarity by a corona discharge, the charge being applied to the back surface being sufficient so that the surface potential of said back surface is more than two times higher than the mean value of the surface potential of said top surface after a succeeding exposure step, said insulating back layer being able to hold the charge being applied to the back surface thereof for a period at least longer than that necessary for completion of developing process; exposing said top surface to a light image; and immersing said photoconductive plate in a liquid developer including carrier liquid and charged toner particles having a concentration in said carrier liquid of between 0.5 mg./cc. and 5 mg./cc. for developing an electrostatic latent image on said top surface.

2. A method of liquid developing a photoconductive plate as claimed in claim 1 wherein a uniform electrostatic charge is given to the top surface of the photoconductive insulating top layer before exposing it to the light image and is given to the back surface of the insulating back layer after exposing it to the light image.

3. A method of liquid developing a photoconductive plate for producing a visible image on the top surface of said photoconductive plate which includes a photoconductive insulating top layer, an intermediate insulating layer which is thinner than said photoconductive insulating top layer, a grounded conductive layer, and an insulating back layer which is thicker than said other layers and is more than ten times thicker than said intermediate insulating layer, comprising the steps of: giving both the top surface of said photoconductive insulating top layer and the back surface of said insulating back layer a uniform electrostatic charge of the same polarity by a corona discharge, the charge being applied to the back surface being suflicient so that the surface potential of said back layer is more than two times higher than the mean value of the surface potential of said top surface after a succeeding step of exposure, said insulating back layer being able to hold the charge being applied to the back surface thereof for a period at least longer than that necessary for completion of developing process; exposing said top surface to a light image; and immersing said photoconductive plate in a liquid developer including carrier liquid and charged toner particles having a concentration in said carrier liquid between 0.5 mg./cc. and 5 mg./cc. for developing an electrostatic latent image on said top surface.

4. A method of liquid developing a photoconductive plate as claimed in claim 3 wherein a uniform electrostatic charge is given to the top surface of the photoconductive insulating top layer before exposing it to the light image and is given to the back surface of the insulating back layer after exposing it to the light image.

References Cited UNITED STATES PATENTS 2,965,481 12/1960 Gundlach 96-1 3,064,259 11/1962 Schwertz 34674 3,379,553 4/1968 Dowley ll7l7.5 3,477,846 11/1969 Weigl et al. 961

GEORGE F. LESMES, Primary Examiner M. B. WITTENBERG, Assistant Examiner US. Cl. X.R. ll737 LX 

